One example of such sensors is the Raman OTDR sensor from which temperature profiles can be determined from the intensity distribution of the backscatter signal measured as a function of time from launching, the probe pulse, or equivalently, distance along the fibre. OTDR technology is used very commonly in telecommunications for assessing the quality and integrity of optical fibre links. In this case, the link is defined by the requirements of the communications system and the OTDR must be designed to cope with the optical fibre system as specified. In the application of OTDR for sensors, however, there is far more flexibility to select the optical fibre system and its interconnection to suit the requirement of the OTDR-sensor designer.
It is known that the performance of OTDR sensors is limited by the power that can be launched into the fibre, owing to the existence of non-linear optical effects. The origin of these non-linear optical effects varies depending upon the characteristics of the probe pulse. For short pulses having a broad spectral width, the dominant effect is usually stimulated Raman scattering (SRS). For longer pulses, with narrower spectral widths, the dominant effect is generally stimulated Brillouin scattering (SBS). For short narrowband pulses, the limiting effect is generally self-phase modulation (SPM), in those cases where the pulse is required to maintain a narrow spectral width. A more detailed explanation of these effects may be found in the literature, for example G. P. Agrawal “Nonlinear fiber” optics Academic Press 1995 ISBN 0-12-045142-5.
The limited power which can be launched into an optical fibre for OTDR-type measurements is a serious limitation on the performance of these systems. The signals received for interrogation pulse in these systems are typically very weak, typically only a few photons are returned in the intended signals from the most remote points of interest in the system. Since the signal received is proportional to the power of the probe pulse, it is clear that an ability to increase the power in the probe pulse will result in a higher performance. Alternatively, if the amount of power that is returned for a given launched power could be increased, then the signal-to-noise ratio could be increased and thus the measurement quality improved. Finally, it should be realised that OTDR measurements are usually averaged over a large number of measurements in order to improve the signal to noise ratio, the latter quantity improving as the square root of the number of measurements averaged. Thus an improvement in the signal-to-noise ratio resulting from a higher power launched can be used to obtain faster update times on the measurement.
It is therefore desirable to provide methods and apparatus to improve the signal-to-noise ratio in OTDR measurements from the section of fibre which is of interest, where this section is separated from the instrumentation by a finite distance. It is frequently the case that the most important section is at the most remote end of the fibre. Whilst the closer sections of fibre may also require to be measured, the present invention concentrates on the improvement of the measurement quality of a remote section of fibre.
The main reason for concentrating on improving measurement at the remote end of the fibre is that this is where the attenuation suffered by the probe pulse in the outbound direction and the signals in the return direction are the highest. There are, moreover, applications where the final section is of primary interest and where the data quality at closer distances is less important. One example of such applications is the measurement of temperature profiles in sub-sea oil wells. In this case, it is important to know the temperature of the fluids flowing in the well to, typically, 0.1° C. However, the equipment is typically located on a platform which is sited some distance away from the well, the well being connected to the platform by a sub-sea flowline which lies on (or is buried within) the sea bed, a riser taking the oil from the flowline up to the platform. The horizontal distance between the platform and the sub-sea well head is known as the “step-out” distance. In a typical example of wells being planned for deep water oil production, the well might extend up to 10 km below the seabed. The step-out distance could be as high as 20-30 km and the water depth can reach 2000 m and may in future exceed this value. In this example, it is the section of optical fibre in the well, the final 10 km, which is important. The fibre which connects the platform with the well head is of lesser importance; it may convey information about blockages in the flowline, but the temperature resolution required for the purposes of flow assurance in the flowline is far less demanding than that required in the well.
In order to provide further background to the invention, the relationship between the maximum power which can be launched in the fibre and the resulting backscatter power will be discussed.
The resolution of the measurand (e.g. temperature) of OTDR-type sensors, such as the Raman OTDR or Brillouin OTDR is generally determined by the signal-to-noise ratio of the backscattered signal. A further discussion of this point may be found in the textbook “Optical Fibre Sensor Technology” edited by Grattan and Meggit (Chapman & Hall, London, 1995, ISBN 0 412 59210 X) and especially in the Chapter on distributed fibre optic sensors. To summarise, the signal returned in such a sensor is proportional to probe pulse energy. In order to increase the pulse energy, either the pulse duration or the pulse power can be increased. In the former case, the spatial resolution (i.e. the ability of the sensor to distinguish closely-spaced features on the profile of the measurand) is degraded. In the later case, the peak power is limited by non-linear effects which convert the probe pulse to different wavelengths from that launched, when the power is increased above certain limits.
In general, it is the intensity of the optical power in the fibre which defines the onset of non-linear effects. By intensity, it is meant the ratio of the optical power launched divided by the area over which this power is spread. Since the optical power is not uniform, an “effective” area is usually defined as follows
      A    eff    =      2    ⁢    π    ⁢                            [                                    ∫              0              ∞                        ⁢                          r              ⁢                                                          ⁢                                                ψ                  ⁡                                      (                    r                    )                                                  2                            ⁢                                                          ⁢                              ⅆ                r                                              ]                2                              ∫          0          ∞                ⁢                  r          ⁢                                          ⁢                                    ψ              ⁡                              (                r                )                                      4                    ⁢                                          ⁢                      ⅆ            r                              
Where φ(r) represents the electric field distribution as a function of the radial coordinate r. The non-linear effects scale inversely with Aeff. It follows that for a given limit at which the non-linear effects become unacceptable, the power which can be launched into the fibre increases in proportion to Aeff. It turns out however, that the backscatter factor, i.e. the ratio of the backscatter signal power to the energy of the probe pulse, is inversely proportional to Aeff. It follows that if the fibre design is changed to increase Aeff, more power can be launched into the fibre, but the fraction of the pulse energy which is converted into a backscatter signal is reduced roughly in proportion to the increase in Aeff, resulting in backscatter signal which is unchanged. Whilst the dopants used to modify the refractive index cause the relationship to deviate somewhat from proportionality with Aeff, the above discussion remains valid, at least approximately. Some of the effects scale further still. For example, for small additions of GeO2 to silica, the core index increases proportionately to the molar concentration of GeO2, leading to an effective area which is reduced as the inverse square of the GeO2 concentration. However, the threshold for stimulated Raman scattering is reduced in a similar proportion because the Raman gain cross section and the spontaneous Raman scattering (from which the stimulated Raman scattering emerges) are both proportional to GeO2 concentration.
The previous paragraph uses terms that relate to single mode fibres, however, it will be understood that the same principles apply to multimode fibres.